The hypothalamic regulation of pituitary gonadotropin secretion and of sexual behavior in the male rat differ from those in the female, and these functional differences are established postnatally under the influence of testicular androgen. This research program is directed at identifying the neural basis of these sex differences in the adult and in elucidating their perinatal development. Recently we have discovered a marked morophological difference within the hypothalamus, the so-called Sexually Dimorphic Nucleus of the preoptic area, and we propose to describe the ontogeny of this anatomical difference and to determine the "birthdates" of the neurons within this nucleus. Moreover, we will attempt to determine whether the hormone environment alters neuronal migration and/or death. In addition the electrophysiological approach of recording the activity of single neurons will be utilized to define developmental changes in hypothalamic connectivity, particularly within the sexually dimorphic region. Finally, we will continue to attempt to elucidate the neurochemical basis for the hormonal facilitation of lordosis behavior in both the normal and hypersensitive septal lesioned female, and in the septal lesioned male subjected to chronic estrogen treatment. This "estrogen sensitization" is required before acute estrogen priming will facilitate lordosis behavior in the septal lesioned male. Aminergic and enzyme activity will be measured following central lesions and the systemic or intracerebral administration of gonadal hormones or amine-active drugs. This broad approach is expected to provide further insight into the mechanisms of the action of gonadal hormones on the developing and adult brain.